Optical maser comprising the active medium cawo4:tm+3



April 1966 L. F. JOHNSON ETAL 3,248,667

OPTICAL MASER COMPRISING THE ACTIVE MEDIUM C WO TM Filed Nov. 20, 1961 ,L. FI JOHNSON 15 K. NASSAU A TTORNEV United States Patent OPTICAL MASER COMPRISING THE ACTIVE MEDllUM CaWOpTm' Leo F. Johnson, North Plainfield, and Kurt Nassau,

Springfield, N.J., assignors to Bell Telephone Laboratories, Incorporated, New York, N.Y., a corporation of New York 4 Filed Nov. 20, 1961, Ser. No. 153,605 7 Claims. (Cl. 33194.5)

This invention relates to optical masers for use in the infrared portion of the optical spectrum. The materials of the invention include a calcium tungstate host lattice and additionally contain thulium ions in the 3+ valence state.

Recently, considerable attention has been focused on a new class of solid state maser devices which are capable of generating or amplifying the coherent electromagnetic wave energy in the optical frequency range. Devices of this type, which are described, for example, in United States Patent 2,929,922 to Schawlow and Townes, are considered to be operable over the spectral range from far infrared to ultraviolet, an equivalent bandwidth of about 10 cycles. Such a bandwidth is capable of providing a great number of new communications channels thereby multiplying the number of available channels which has heretofore been limited by the characteristics of the lower frequency portions of the spectrum.

Typically, a maser of the type known in the art employs an active material characterized by a plurality of distinct energy levels, the separation of these levels corresponding to frequencies within the desired operating frequency ranges. More particularly, the separation between two of the energy levels E and E corresponds to quantized wave energy having a frequency v given by Bohrs equation where h is Plancks constant.

In accordance with the maser principle, wave energy in an appropriate frequency range is applied to an ensemble of paramagnetic ions, thereby pumping electrons from a lower energy level to a metastable higher level. The excited electrons tendto remain in the upper level for a short time before decaying or relaxing to the lower level. The downward transition is normally accompanied by the radiation of wave energy of the frequency corresponding to separation between the energy levels concerned. During maser operation sufiicient pump power is supplied to the active medium to produce, at least I intermittently, a nonequilibrium population distribution between the pair of energy levels. More particularly, the population of the upper level is increased with respect to that of the lower level. When the population of the upper level exceeds that of the lower level, a population inversion or negative temperature is considered to obtain. Operation of the maser is dependent upon the fact that a small signal at the proper frequency acts to stimulate the downward transition of the excited electrons from the metastable state, and that the stimulated emission is coherent and in phase with the signal.

Among the more promising forms of maser are those which utilize an active medium characterized by first, second, and third successively higher electron energy levels. Typically, in optical maser materials the upper level is relatively broad, and in some cases is best described as a band. Continuous wave operation of such three-level devices may be achieved, for example, by pumping electrons from the first to the third level from which they relax spontaneously, by nonradiative processes, to the second level, thereby producing the desired population inversion between the second and first levels. Advantageously, in masers of this type, the relaxation time between the third and second levels is shorter than that between the second and first levels so that the population of the second level may be continuously maintained during operation of the device. Additionally, as the magnitude of the negative temperature attained depends on the relative populations of the first and second levels, the energy level system of preferred materials also include mechanisms which continuously depopulate the terminal state of the optical transition. Thus, the population inversion is maintained at a relatively high value and maser action is facilitated.

Among the more promising active maser materials are those which comprise a host crystal containing paramagnetic ions from which the stimulated emission occurs. The host crystal must be of a material capable of accepting the paramagnetic ions in such a way that they are able, upon excitation, to fiuoresce with good over-all quantum efiiciency, with as much as possible of the emitted energy concentrated in a single narrow spectral line.

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, More particularly, the host must accept the ions in such a way as to minimize coupling between them and the crystal lattice at the maser frequency, while simultaneously permitting relaxation from the pump band to the metastable state, Additionally, the host crystal should be of good optical quality. That is to say, it must be relatively free of scattering centers and hence transparent to light waves at the operating frequency of the maser. Furthermore, the host should have a low coefiicient of absorption at the pump frequency to minimize heating of the maser medium and to promote more eflicient utilization of the pump power. Chemical and physical stability are further desiderata. It is also required that the host be a mechanically workable material, capable of being accurately shaped and highly polished.

A combination of paramagnetic ions with a host lattice meeting the above-mentioned conditions is ruby, which continues to be widely used as an optical maser medium and is, in fact, one of the very few materials which have been operated successfully. Ruby has usable emission lines at .6943 micron and .6921 micron. In addition, ruby having a high concentration of chromium ions is characterized by sharp satellite lines at .7009 micron and .7041 micron. Another successful material comprises a calcium fluoride host lattice containing samarium ions. Emission from the samarium ions is at .7082 micron. A more recently discovered optical maser medium, disclosed in copending application Serial No. 139,266 filed September 19, 1961 of Johnson and Nassau,.is calcium tungstate containing trivalent neodymium ions. Additional newly discpvered materials are disclosed in copending applications Serial Nos. 153,603, 153,606, 153,607 and 153,604 all filed concurrently and assigned to the assignee hereof. These include praseodymium in calcium tungstate operating at about 1.047 microns; neodymium in strontium molybdate operating at about 1.06 microns; holmium in calcium tungstate operating at about 2.046 microns; and neodymium in calcium fluoride operating at about 1.046 microns.

Ruby optical masers of the usual design are subject to the disadvantage of requiring a high pump power to establish the required population inversion. The amount of power supplied to the ruby and the conditions of its absorption by the crystal have thus far limited ruby maser operation to producing a pulsed beam of coherent light. It is apparent, however, that in many applications it is highly desirable to produce continuous coherent light beams by maser action.

Furthermore, it is to be noted that the choice of active medium for an optical maser device governs the frequency of the usable emission lines. Thus, it is desirable to-provide a variety of optical maser materials in order to make possible the generation and amplification of coherent light beams over the wide range of the optical frequency spectrum. However, despite the discovery of several operable materials, on the basis of present information there is no known theory which is able to predict which combinations of ions and host lattices will be successful.

An object of the invention is the generation and amplification of coherent radiation in the infrared portion of the optical frequency spectrum.

It is also an object of the invention to provide an optical maser capable of operation in the infrared range and requiring a relatively low pumping power.

In accordance with this invention, a new fluorescent material suitable for use in optical masers has been discovered. This material consists of a calcium tungstate host lattice in which some of the calcium ions have been replaced by thulium ions in the 3+ valence state. Trivalent thulium is the active maser medium and stimulated emission is produced at about 1.9 microns in the infrared region. At 77 degrees K., the temperature of liquid nitrogen, stimulated emission is produced at 1.91 microns.

A feature of the invention is an optical maserhaving an active medium consisting of trivalent thulium ions in a calcium tungstate host crystal.

An optical maser in accordance with the invention is shown in the figure. There is depicted a rod-shaped crystal 1 of calcium tungstate having an appropriate small concentration of trivalent thulium ions as disclosed herein. Pump energy is supplied by a helical lamp 2 encompassing rod 1 and connected to an energy source not shown. Ends 3 and 4 of rod 1 are ground and polished in the form of confocal spherical surfaces. Reflective layers 5 and 6 are deposited on ends 3 and 4, thereby forming an optical cavity resonator of the type described in a copending patent application Serial No. 61,205, filed October 7, 1960, by Boyd, Fox and Li, issued September 25, 1962, as United States Patent No. 3,055,257. Advantageously, layer 6 is highly reflecting while layer 5 includes at least a portion which is only partially reflecting to permit the escape of coherent radiation 7 having a wavelength of about 1.9 microns. It is preferred that, during operation of the maser the rod 1 be maintained in a bath of a liquefied gas, such as nitrogen, to maintain a low temperature with the desirable results hereinafter described.

The lamp 2 is advantageously of a type which produces intense radiation over a broad band extending from about 0.3 micron to longer wavelengths. Mercury or xenon lamps are considered useful to pump the material of the invention, which is characterized by a plurality of very sharp absorption lines in the specified spectral range. Other types of lamps may, of course, be employed provided they emit sufiicient energy at wavelengths corresponding to one or more useful absorption lines of the material. Electrons in the active medium are excited to upper energy levels by the pump power and relax through nonradiative processes to an intermediate level corresponding to one of the H level of free thulium ions. This level corresponds to the metastable level of the above-mentioned exemplary three-level system. A negative temperature is thus created between this level and one of the H levels. The H level lies about 325 cm.- above the ground state and has a negligible population at room temperature. The population of the terminal state may be further released by cooling the crystal. Stimulated emission at 1.91 microns in the infrared is produced by this device.

An optical maser of the type illustrated in the figure has been operated using as an active medium calcium tungstate containing about 0.1 atomic percent of thulium in place of calcium. The device produced intense emission at 1.91 microns in the infrared. Maser action may be achieved over a range of thulium concentration. Al-

though in principle there is no lower limit on the concentration of T m+ which may be employed in the crystal 1, yet a practical limit of about 0.01 percent is imposed by the necessity of having sufficient unpaired electrons available to produce a reasonable output.

Additionally, the preferred concentration of Tm+ ions in the CaWO host crystal is deemed to be about 1 percent. Such a concentration appears desirable from the standpoint of maximum intensity in the narrowest possible line. However, effective concentrations are deemed to extend as far as 3 percent Tm+ Beyond 3 percent increasing account must be taken of line broadening due to interaction among the thulium ions themselves. Although stimulated emission may be obtained with concentrations as high as 10 percent, most applications will require a maximum of about 3 percent. Furthermore, crystals having more than about 3 percent thulium ions are somewhat more difficult to manufacture than are those having smaller concentrations.

From measurements made on the device described above, it is estimated that, at liquid nitrogen temperature, the power required to operate a maser in accordance with the invention is about 5 percent of that required to operate a ruby maser of similar configuration under the same condition. Active maser media based on samarium for example, while operable at liquid hydrogen temperature, are not operable at liquid nitrogen temperature.

The material of the invention is conveniently made by a method generally designated as the Czochralski method. Reference may be made to an article by J. Czochralski in Z. Physik, Chemie, vol. 92, pages 219221 (1918). A recent description of the process is to be found in an article by K. Nassau and L. G. Van Uitert in Journal of Applied Physics, vol. 31, page 1508 (1960). Among the other techniques which may be employed are the flame fusion, hydrothermal flux and spontaneous nucleation techniques.

A particular sample of the substance was prepared by placing 200 grams of CaWO in an iridium crucible and melting it. 0.30 gram of Tm O and 0.54 gram of W0 were then added to the melt and a homogeneous mixture produced. A seed crystal of the desired orientation was inserted into the top surface of the melt and was simultaneously rotated and drawn from the melt. Good results were obtained when the speed of rotation was about 30 rpm. and the rate of pulling or drawing was about /2 inch per hour. The resulting calcium tungstate crystal contained about 0.08 percent thulium ions in place of calcium.

It will be noted that a considerable excess of thulium ions was added to the melt. In general, it has been found that the concentration of thulium in the crystal is about One-third of that in the melt. Thulium may be also added in various other forms such as Tm (WO or NaTmW O Although the invention has beendescribed with reference to a specific embodiment, this is to be construed by way of illustration and does not limit the scope of the invention. For example, the material of the invention may be used with any concentration of thulium in the ranges set forth. Furthermore, the material may be used in optical cavity resonators other than the confocal type. The parallel plate resonator, as well as others, may also be employed. Other variations are also possible within the spirit of the invention.

What is claimed is:

1. An optical maser material consisting essentially of a crystalline calcium tungstate host lattice in which a portion of the calcium ions have been replaced by thulium ions in the trivalent state, the portion of calcium ions so replaced being in the range of from about 0.01 percent to about 10 percent.

2. An optical maser material consisting essentially of a crystalline calcium tungstate host lattice in which a portion of the calcium ions have been replaced by thulium ions in the trivalent state, the portion of calcium ions so replaced being in the range of from about 0.01 percent to about 3 percent.

3. An optical maser comprising a negative temperature medium consisting essentially of the material claimed in claim 1, means for producing a population inversion between a pair of optically connected energy levels of said thulium ions, and means for stimulating coherent emis sion at the Wavelength corresponding to the separation of said levels.

4. An optical maser comprising a negative temperature medium consisting essentially of the material claimed in claim 2, means for producing a population inversion between a pair of optically connected energy levels of said thulium ions, and means for stimulating coherent emission at the wavelength corresponding to the separation of said levels.

5. An optical maser comprising a negative temperature medium consisting essentially of a calcium tungstate host crystal in which about 0.1 percent of the calcium ions have been replaced by thulium ions in the trivalent state, means for producing a population inversion between a pair of optically connected energy levels of said thulium ions, and means for stimulating coherent emission at the wavelength corresponding to the separation of said levels.

6. An optical maser as claimed in claim 5 and further comprising means forming an optical cavity resonator, and means for abstracting said coherent emission from said resonator.

7. An optical maser comprising a rod shaped substantially monocrystalline negative temperature medium consisting essentially of calcium tungstate in which about 0.1 percent of the calcium ions have been replaced by trivalent thulium ions, said rod having confocal spherical end surfaces, said end surfaces being covered with a reflective layer thereby forming an optical cavity resonator in said medium, pumping means for producing in said rod a population inversion between a pair of optically connected energy levels of said thulium ions, and means for abstracting from said cavity coherent emission of a wavelength corresponding to the energy separation of said levels.

References Cited by the Examiner UNITED STATES PATENTS 2,929,922 3/ 1960 Schawlow et a1 881 3,055,257 9/1962 Boyd et a1 881 OTHER REFERENCES Kroger: Some Aspects of the Luminescence of Solids, Elsevier Publishing C0., Inc., New York, 1948, pages 288-298.

TOBIAS E. LEVOW, Primary Examiner.

JULIUS GREENWALD, MAURICE A. BRINDISI,

' Examiners. 

1. AN OPTICAL MASER MATERIAL CONSISTING ESSENTIALLY OF A CRYSTALLINE CALCIUM TUNGSTATE HOST LATTICE IN WHICH A PORTION OF THE CALCIUM IONS HAVE BEEN REPLACED BY THULIUM IONS IN THE TRIVALENT STATE, THE PORTION OF CALCIUM IONS SO REPLACED BEING IN THE RANGE OF FROM ABOUT 0.01 PERCENT TO ABOUT 10 PERCENT.
 3. AN OPTICAL MASER COMPRISING A NEGATIVE TEMPERATURE MEDIUM CONSISTING ESSENTIALLY OF THE MATERIAL CLAIMED IN CLAIM 1, MEANS FOR PRODUCING A POPULATION INVERSION BETWEEN A PAIR OF OPTICALLY CONNECTED ENERGY LEVELS OF SAID THULIUM IONS, AND MEANS FOR STIMULATING COHERENT EMISSION AT THE WAVELENGTH CORRESPONDING TO THE SEPARATION OF SAID LEVELS. 